The invention relates to membrane heat engines utilizing a saturated solar pond as an unmixing device in the generation of power from the mixing of a dilute and concentrated solution through either a pressure-retarded osmosis (PRO) permeator, or by reverse electrodialysis (RED) permeator. Both the PRO permeator and the RED permeator are, of themselves, known, and saturated solar ponds are also known. All of them are today experimental and developmental, not having reached the stage of successful commercial exploitation. For example, in U.S. Pat. No. 3,906,250 to Loeb, are described various pressure-retarded osmosis heat engines. One variation shows a solar pond, (no reference to saturation), used as a heat source only, rather than as an unmixing device. At another point, thermal unmixing by use of a solute whose solubility is a function of temperature is disclosed, but reference is to filtration of a precipitated solute, i.e., not unmixing completely in liquid phase as described herein.
Reverse Electrodialysis or the Dialytic Battery has been disclosed in numerous publications, Weinstein & Leitz Science 191, 557 (1976); Energy by Reverse Electrodialysis, S.R.I. report to D.O.E. (1978). All of these prior disclosures have assumed an open system with fresh water and brine input, a discharge and no recirculation.
In obtaining useful power from PRO, potential energy is obtained by the permeation of a solvent, such as water, into a pressurized brine solution, such as one of ordinary salt, thereby increasing the volume of the brine solution under pressure. The resultant increased volume under pressure may be employed to do useful work and thereby derive energy from the unmixing. In RED, electrical energy is obtained directly due to the transport of cations or anions through membranes permeable to either one or the other ion. Electrodes are then employed in different chambers to connect the resultant electrical current to an external circuit to perform useful work.
The saturated solar pond, as described in U.S. patent application Ser. No. 958,037 filed Nov. 6, 1978, entitled "Solar Collection System," has the interesting characteristic, as an unmixing device, that it uses unfocused solar energy to divide a spent solution into a concentrated and dilute solution. Furthermore, it accomplishes this unmixing without precipitation. It combines within itself the function of a heat source, heat sink, and unmixer. Thus the saturated solar pond (SSP), when used together with a membrane permeator and attendant equipment, comprises a novel heat engine for efficient production of useful energy from low grade solar energy.